Numerical Study on the Effects of Diluents on the Laminar Burning Velocity of Methane-Air Mixtures

The effect of diluents on the laminar burning velocity of the premixed methane-air-diluent flames was numerically studied using the Chemkin package. The mechanisms of dilution, thermal-diffusion, and chemical effects of diluents on the laminar burning velocity were analyzed quantitatively at different dilution ratios for different diluents. Results show that the laminar burning velocity is decreased in the order from helium, argon, nitrogen, and carbon dioxide. In the case of N-2, the thermal-diffusion and chemical effects can be negligible and the decrease of the laminar burning velocity is largely caused by the dilution effect. The dilution, thermal-diffusion, and chemical effects of CO2 suppress the laminar burning velocity, where the dilution effect plays a dominant effect among them. For helium and argon diluents, the chemical effect can be negligible and the thermal-diffusion effect enhances the laminar burning velocity. Therefore, the dilution effect has a much larger suppression effect on decreasing the laminar burning velocity to counteract the thermal-diffusion effect of helium and argon. An empirical formula of the laminar burning velocity that takes into account the adiabatic flame temperature and thermal diffusivity is obtained. Good correlations between the laminar burning velocity and mole fraction of H + OH at the position of the maximum mole fraction of the H radical in the flame are also demonstrated. The laminar burning velocity has the same tendency with the product of thermal-diffusion and chemical reaction terms as a function of the dilution ratio for different diluents. The adiabatic flame temperature plays a dominant influence on the laminar burning velocity, and thermal diffusivity has a secondary influence on methane-air-diluent flames.